Readers first become acquainted with Sarah Pocket in Chapter 11, when Pip arrives at Miss Havisham's and is instructed by Estella to wait in the corner of the room until he is called for. In the same room, Sarah Pocket, along with three others, are also waiting for Miss Havisham. When Pip is called into the room first, Sarah Pocket takes offense, saying, "well I am sure! What next!"
When Sarah Pocket and the others are finally allowed to see Miss Havisham, readers understand that they are relatives of hers who visit her regularly with the hopes that Miss Havisham will include them in her will. In a ridiculous scene, Miss Havisham asks Pip to parade her around her great table and shows him where she will be laid when she is dead. Later, she dictates who will sit where at the table (Sarah Pocket is assigned a spot) to "feast upon" her.
Later in the novel, when Sarah Pocket realizes that Pip is still part of Miss Havisham's life, she becomes even more resentful, as she assumes that Pip will somehow compromise the amount of money she will receive from Miss Havisham. In Chapter 29, Pip observes evidence of this during a dinner at Miss Havisham's:
Throughout dinner, [Jaggers] took a dry delight in making Sarah Pocket greener and yellower by often referring in conversation with me to my expectations."
Though neither Pip nor Sarah Pocket knows who is funding Pip's education, both assume it is Miss Havisham. Obviously, Pip understands the source of Sarah Pocket's hatred of him:
I think Miss Pocket was conscious that the sight of me involved her in the danger of being goaded to madness, and perhaps tearing off her cap.
Obviously, Sarah Pocket serves as Dickens's reminder to readers that relationships built around money--or the hopes of receiving it--are most often shallow, meaningless ones.